Saturation of absolute beam-plasma instability by beam trapping

The first experimental observation of saturation of the absolute instability due to beam trapping is reported. The details of the trapping differ from the convectively unstable case in that the wave is large near the electron gun, and the beam is suddenly trapped in the large amplitude wave. The experimental results are in agreement with a calculation of the non-linear beam dynamics, based on the trapping model, but modified to include a backward plasma wave.